Earthlearningidea - http://www.earthlearningidea.com/ Sedimentary structures – cross-bedding and ‘way-up’ Using cross bedding to determine the way-up of a bed of sedimentary rock

When water flows over loose sand, small-scale We can use these observations to work out if a dunes can form like those shown in photograph 1. cross-bedded sandstone is the right way up, or if it has been turned upside down by Earth movements (or when it is used in a building). Explain this principle to the class, using Photograph 2.

Truncation plane

Photo 1: Subaqueous dunes formed on a beach; the dunes Upward curving were formed by tidal currents flowing out to sea on a falling cross beds tide. (Photo: Chris King)

These subaqueous dunes are around half a metre Photo 2: Cross-bedding in sandstones, Eday, Orkney Islands. high and several metres wide and so are much The beds are the right way up. (Photo: Peter Kennett) smaller than wind-formed dunes. After being Now ask the class to say if the block of coarse formed by the tide, they are often destroyed, and so are not often seen on beaches, but the sandstone in Photograph 3 is the right way up or if it is inverted. (It is the right way up, since the important thing is that they produce cross- truncation plane cuts sharply across above cross- bedding, as described below – and this is often beds). preserved in sandstones.

As water currents flow across sand in beaches, rivers, etc., subaqueous dunes form and then are moved steadily forward. The water moves the sand up the dune until it reaches the top and then the sand avalanches down the other side in a sheet of sediment; this slopes downward in the same direction as the current, at around 20o. These sloping layers of sand are called cross- bedding.

The diagram below shows that a) the lower slopes of the subaqueous dunes are often gently curved upwards; b) as a new dune advances, it truncates (cuts off) the top of the previous one. Photo 3: Cross bedding in a coarse sandstone, Burbage Edge, Sheffield (Photo: Peter Kennett)

This method can be practised on building stones. Current Show pupils Photographs 4 and 5, which show two different parts of the same building, constructed from red sandstone. Ask them to work out which block of sandstone is the right way up and which has been placed upside down by the builders. (The block in Photo 4 is upside down – the truncation plane cuts downward-curving cross- beds, just below the coin. The block in Photo 5 is the right way up – the cross beds curve gently

Diagram showing the relationship between current flow and upwards) the structure of the dunes.

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Photo 4: Red sandstone on the front of a building (Coin = 2cm) Photo 5: Red sandstone on the front of the same building (Photo: Peter Kennett) (Coin = 2cm) (Photo: Peter Kennett)

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The back up Title: Sedimentary structures – cross-bedding top down by avalanching, as in dunes). Their and ‘way-up’ internal structure is similar to cross-bedding, but on a smaller scale, and is called cross-lamination. Subtitle: Using cross bedding to determine the way-up of a bed of sedimentary rock The formation of these features may be observed in modern depositional environments and then Topic: An introduction to the types of evidence related to similar structures in sedimentary rocks – which can be obtained from cross-bedding in an example of using the present to help sediments and in sedimentary rocks. understand the past.

Age range of pupils: 14-18 years Following up the activity: Use Earthlearningidea activity Sedimentary structures – cross-bedding Time needed to complete activity: 10 minutes and ancient currents: using cross-bedding to determine the direction of ancient currents, to Pupil learning outcomes: Pupils can: show how cross-bedding may also be used to • identify cross-bedding in sediments and in determine ancient current directions, with possible sedimentary rocks from photographs; economic connotations. Look out for examples of • relate cross-bedding to the original conditions cross-bedding in buildings and work out which of deposition of the sediment; way up the blocks are. • use cross-bedding to determine the way-up of a series of sedimentary rocks; Underlying principles: • understand that cross-bedding in building • Sediment of grain size coarser than silt, such stones may be used to determine if the block as sand, is not always deposited in horizontal has been placed upside down or the right way layers. up. • Currents wash sand grains along in the direction of the current. Context: Cross-bedding is a common feature of • Variations in the current lead to underwater sedimentary rocks. It can be formed by both dunes forming at regular intervals on the river subaqueous dunes (as described here) and dunes bed or the sea bed. formed by wind (eg. in deserts and on coasts). As • Sand grains ride up the backs of these dunes aeolian (wind-formed) dunes are metres in height and then avalanche down the front side, to and tens of metres across, the cross bedding come to rest at an angle from the horizontal, at formed by them is also larger, with single sets of 20o or so. cross-beds often being more than a metre thick. Thinking skill development: Identifying patterns Asymmetrical ripple marks have similar shapes to in cross-bedding and relating them to the former dunes, but are on a smaller scale of centimetres current involves skills of construction. rather than metres. They are also formed by both Metacognition is involved when pupils discuss water currents and air currents, but in a slightly which way up the sandstones are, as seen in the different way to dunes (sand is deposited from the photographs. Linking observations from bottom up on the sloping surfaces, not from the photographs to the real world involves bridging.

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Earthlearningidea - http://www.earthlearningidea.com/ http://www.nationalstemcentre.org.uk/elibrary/reso Resource list: urce/721/unit-4-building-sedimentary-structures • copies of the photographs and the diagram • (Optional) specimens of cross-bedded Source: Written by Peter Kennett of the sedimentary rocks, or access to a building Earthlearningidea team. constructed from them.

Useful links: http://www.nationalstemcentre.org.uk/elibrary/reso urce/734/second-hand-rocks-introducing- sedimentary-processes and

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